MP2497 – 3A, 50V, 100kHz STEP-DOWN CONVERTER

MP2497 Rev. 1.12

www.MonolithicPower.com

10

12/22/2011

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© 2011 MPS. All Rights Reserved.

Selecting the Inductor

A 10µH to 47µH inductor with a DC current

rating of at least 25% percent higher than the

maximum load current is recommended for

most applications. For highest efficiency, the

inductor DC resistance should be less than

200mΩ. For most designs, the inductance value

can be derived from the following equation.

OUT

IN

OUT

IN

L

OSC

V(V

V

)

L

VI

f

×−

=

×Δ ×

(9)

Choose

inductor

current

ripple

to

be

approximately 30% of the maximum load

current, 3A. The maximum inductor peak

current is:

L

L(MAX)

LOAD

I

II

2

Δ

=+

(10)

Under light load conditions below 100mA, larger

inductance is recommended for improving

efficiency.

Selecting the Input Capacitor

The input capacitor reduces the surge current

drawn from the input and also the switching

noise from the device. The input capacitor

impedance at the switching frequency should

be less than the input source impedance to

prevent high frequency switching current from

passing to the input. Ceramic capacitors with

X5R

or

X7R

dielectrics

are

highly

recommended because of their low ESR and

small

temperature

coefficients.

For

most

applications, a 4.7µF capacitor is sufficient.

Selecting the Output Capacitor

The output capacitor keeps output voltage small

and ensures regulation loop stability. The

output capacitor impedance should be low at

the switching frequency. Ceramic capacitors

with X5R or X7R dielectrics are recommended.

Setting the Output OVP Threshold

The output OVP threshold is set by connecting

an external resistor divider (R3, R4 see the

typical application circuit on the front page) at

OVP pin. Choose R3 to be 39kΩ for lower

power dissipation. Then, R4 is given by:

OVP

OVREF

R3

R4

(k )

V

1

V

=

Ω

−

(11)

Setting the Current Limit

The hiccup current limit can be set by the DC

resistance (DCR) of the inductor, as shown in

Figure 3a.

For more accurate sensing, use a more

accurate sense resistor.

In Figure 3a, the output current limit is set as:

OUT _ L

100mV

I

DCR

=

(12)

Where, DCR is the DC resistance of the

In Figure 3b, the output current limit is set as:

OUT _ L

SENSE

100mV

I

R

=

(13)

SW

ISP

ISN

SW

ISP

ISN

(a)

(b)

Figure 3—Current Sensing Methods